A novel barophilic, hyperthermophilic, anaerobic sulfur-metabolizing archaeon, strain MPT(T = type strain), was isolated from a hydrothermal vent site (Snakepit) on the Mid-Atlantic Ridge (depth, 3550 m). Enrichments and isolation were done under 40 MPa hydrostatic pressure at 95°C. Strain MPTwas barophilic at 75, 80, 85, 90, 95 and 98°C, and was an obligate barophile between 95 and 100°C (Tmax). For growth above 95°C a pressure of 15·0-17·5 MPa was required. The strain grew at 48-95°C under atmospheric pressure. The optimal temperature for growth was 85°C at both high (40 MPa) and low (0·3 MPa) pressures. The growth rate was twofold higher at 85°C under in situ hydrostatic pressure compared to at low pressure. Strain MPTcells were motile, coccoid, 0·8-2·0 μm in diameter and covered by a hexagonal S-layer lattice. The optimum pH and NaCI concentration for growth at low pressure were 7·0 and 20-30 gl-1, respectively. The new isolate was an obligate heterotroph and utilized yeast extract, beef extract and peptone for growth. Growth was optimal in the presence of elemental sulfur. Rifampicin and chloramphenicol inhibited growth. The core lipids consisted of a major archaeol and a complex lipid pattern consisting of a major phospholipid. The DNA G+C content was 37·1 mol%. Sequencing of the 16S rRNA gene revealed that strain MPTbelonged to the genus Thermococcus and it is proposed that this isolate should be designated as a new species, Thermococcus barophilus.

An autotrophic, hyperthermophilic methanogen (M7T) was isolated from a deep-sea hydrothermal chimney sample collected on the East Pacific Rise at depth of 2600 m. The coccoid-shaped cells are flagellated and exhibit a sligh tumbling motility. The temperature range for growth at pH 6.5 was 49--89 °C, with optimum growth at 80 °C. The optimum pH for growth was 6.5, and the optimum NaCI concentration for growth was around 25 g I-1. The new isolat used H2 and CO2 as the only substrates for growth and methane production. Tungsten, selenium and yeast extract stimulated growth significantly. In the presence of C02 and H2 the organism reduced elemental sulphur to hydrogen sulphide. Growth was inhibited by chloramphenicol and rifampicin, but not by ampicillin, kanamycin, penicillin and streptomycin. The G+C content of the genomic DNA was 31 mol%. As determined by 16S rDNA gene sequence analysis, this organism was closely related to Methanococcus jannaschii strain JAL-1T. However, despite the high percentage of similarity between their 16S rDNA sequences (97.1 %), the DNA-DNA hybridization levels between these strains were less than 5%. On the basis of these observations and physiological traits, it is proposed that this organism should be placed in a new species, Methanococcus vulcanius. The type strain is M7T(= DSM 12094T) During the course of this study, the 16S rDNA sequence analysis placed Methanococcus sp. strain AG86T(= DSM 4213T) as a close relative of M. jannaschii strain JAL-1T. However, the weak level of DNA-DNA hybridization with this strain (< 10%) allowed the proposal that strain AG86Talso constitutes a new species, Methanococcus fervens.